And a security researcher warned there might be another attack
imminently.

Wainwright said: "We're in the face of an escalating threat, the
numbers are going up, I'm worried about how numbers will continue
to grow when people go to work and turn their machines on on
Monday morning."

Europol is the EU's law enforcement and intelligence agency. It's
working with the FBI to track down the criminals responsible for
the malware, but Wainwright said this was "very difficult."

"We have never seen anything like this," he said. "We've seen the
rise of ransomware becoming the principal cyber threat, but this
is something we've never seen before — the global reach is
unprecedented."

At least 48 NHS organisations were affected by the hack,
including St. Bartholomew's Hospital, and the East and North
Hertfordshire Trust. Staff resorted to working with pen and paper
while their computer systems were down, and hospitals had to
cancel appointments.

Barts Health, the biggest NHS trust, said it was still
experiencing issues as of Sunday.

It's likely more NHS organisations will be hit on Monday, because
the attack began late on Friday.

“Given the timing of the cyber-attack, some parts of the NHS will
not have clocked there is an issue,” a spokeswoman for NHS
Digital told the Guardian.
“If that is going to happen it is more likely to be primary care
trusts.”

The attack took the form of ransomware that is nicknamed
"WannaCry". Ransomware is malicious software that encrypts data
on your computer, then asks for payment in return for decryption.
In this case, messages seen by affected NHS staff showed that the
attackers were asking for $300 in Bitcoin in exchange for
decryption.

Some security experts reported that the ransomware spread so
quickly because it was updated with Windows exploits created by
the US National Security Agency. These were leaked online — and
subsequently made their way into the hands of criminals.

US President Donald Trump ordered his homeland security
adviser, Tom Bossert, to hold an emergency meeting Friday night
to assess the threat,
Reuters reported. The FBI and the NSA, along with
intelligence agencies across the globe, are working to identify
the perpetrators of the massive cyber attack.

The NHS is running old, insecure computer software

But the attack has sparked a massive debate about why the NHS, as
a critical piece of the UK's infrastructure, was running
out-of-date software at all.

Europol's Wainwright said organisations should learn from the
banking sector, which had learnt the "painful" way to keep
systems updated.

"They have learnt through painful experience of being the number
one target for cybercrime of the value of having a proper
strategy in place," he said, "and I think the health sector and
others should follow the example to make sure they sit up and
take notice of what is absolutely a huge strategic concern."